This invention relates to locking means for a piston pin which is loosely mounted in the bosses of a piston, which piston is larger in outside diameter than the external distance between the bosses.
The piston pin connects the piston and the connecting rod and must transmit the forces which act between these members. The outside dimensions of the piston pin are determined by the dimensions of the piston and the strength properties of the pin material.
To prevent contact between the piston pin and the cylinder wall during the operation of the engine, the piston pin must be locked against axial movement. Such lock will be inherently provided if the piston pin is clamped to the connecting rod. In connection with floating piston pins, which are loosely mounted in the bosses and in the connecting rod small end bearing, it is conventional to use locking means which consist of round wire or sheet steel and which are externally mounted on both ends of the piston pin and snap into registering locking grooves provided on the periphery of the bosses. Locking rings tending to expand radially are usually employed. The locking rings may particularly consist of circlips in accordance with DIN 472 or of wire snap rings in accordance with DIN 73 120. The manufacturing tolerances result in an axial play of the piston pin between the two locking rings mounted in the bosses. In small pistons this play is about 0.4 - 0.1 mm.
It is also known to insert a mushroom-shaped light-alloy element into the bore of the piston at each open end thereof so that said element extends along the cylinder wall.
The snap rings and circlips must be compressed and must then be inserted into the bosses until the rings snap into the circumferential grooves formed in the bosses. Such rings must have the required diameter even when they have been compressed several times. An additional disadvantage which must be tolerated is that the external distance between the bosses must exceed the length of the piston pin so that the space required for the locking rings or mushroom-shaped locking elements is available in each boss. Where locking rings are used, it is necessary to cut grooves into the bosses and also to clean the grooves before the locking rings are inserted so that a uniform contact is ensured and a subsequent loosening of the locking rings in operation is precluded. Besides, the rings can be inserted only with special pliers in order to ensure that the locking ring will not suffer permanent deformation before it is inserted. It is apparent that the manufacture of the piston pin-locking means and the insertion of such means involve a comparatively large expenditure.